In Magnetic Resonance (MR) imaging applications, the term field-of-view (FOV) refers the distance over which an image is acquired or displayed. Recently, a MR acquisition technique has been proposed for extending the FOV by implementing an MR sequence that uses a readout gradient field to compensate B0 field inhomogeneity and gradient nonlinearities. The method, called B0 homogenization using gradient enhancement (HUGE), can be applied in the readout direction in which the distortions depend on both the gradient nonlinearities and on the B0 inhomogeneity (as opposed to the phase-encoding direction in which they depend only on the gradient nonlinearities). HUGE allows the extension of the FOV to 60 cm (i.e., equal to the bore size of a typical MR scanner).
There were several limitations of the HUGE technique. For example, the optimal readout gradient must be determined for each side of the subject (i.e., left and right arm) at each bed position. This substantially increases the total acquisition time. Moreover, the typical gradient linearity region for a clinical system is 50 cm, which truncates the arms of the patient and, in the case of large body habitus, large regions of the anatomy. Generating images of these truncated portions of the anatomy can be critical to both calibration and reference scans which rely on patient specific settings, such as parallel transmit MR and simultaneous MR-PET. Generating images of these truncated portions of the anatomy can be critical to both calibration and reference scans which rely on patient specific settings, such as parallel transmit MR, simultaneous MR-PET and radiation oncology therapy planning. Hybrid applications that combine modalities require accurate patient specific body models that account for the correct anatomical positioning in the MR scanner. Specifically, for advanced parallel transmit pulse design, local SAR deposition depends greatly on patient specific anatomy. It is a similar case with radiation therapy planning where the dose strategy is modulated by the patient's tissue structure. The PET imaging example focusing on the reverse problem where radiation coming from the body is attenuated by the surrounding tissue.